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Hi, I'd be happy to help you with this question!
3.1 mol - 3 mol = 0.1 mol A mol (mole) is equal to 6.022x1023 atoms so 0.1 mol = 6.022x1024
As to the other one, 51.2 g - 16 g = 35.2 g
Significant digits are just those numbers that carry meaning in the number. That means that things like an extra zero at the end or numbers that are bigger than the original value. That last one means that if a number is XXXXX one decimal place (like the questions) you would only carry the answer to the same number of decimal places. (I'm not sure in the first one using moles if your teacher is asking for the atoms or the moles and if it's the latter I'm not certain how many decimal places you would need to carry it to. I think that they just want the number of mols though, assuming that this is a lesson on significant figures.)
Does that answer your question and explain it adequately?